The invention relates to a switched-mode power supply, and a display apparatus comprising such a switched-mode power supply.
In present day applications, especially in standby modes of apparatus, low power consumption is getting more and more crucial due to government legislation and consumer demand. The preliminary specification of the TNY253/254/255 Tinyswitch Family, Energy efficient, low power Off-line switchers of Power Integrations, Inc, June 1998 discloses a switched-mode power supply chip which is able to operate in a burst mode to obtain a high efficiency at low output power.
The disclosed switched-mode power supply comprises a four-terminal integrated circuit (further referred to as chip) which comprises a MOS field-effect transistor (further referred to as FET) with a main current path arranged between a first and a second terminal of the chip, a regulator arranged between the first terminal and a third terminal of the chip, a comparator for comparing a voltage at the first terminal with a reference voltage, and a control circuit. The control circuit comprises an oscillator, a set-reset flip-flop, several logic gates, and an under-voltage lock-out detection circuit (further referred to as under-voltage detector) for comparing a voltage at the third terminal with a further reference voltage. The detailed construction of the control circuit will be described later. The switched-mode power supply further comprises a primary winding of a transformer which is connected to the first terminal to obtain a series arrangement with the main current path of the FET. The series arrangement receives a DC input voltage. A diode is connected to a secondary winding of the transformer to generate a DC output voltage. A capacitor is connected to the third terminal of the chip. The regulator generates a voltage across the capacitor. This voltage across the capacitor is used as a supply voltage for other circuits in the chip. A feedback circuit comprises an optocoupler and receives the DC output voltage to supply a signal to a fourth terminal of the chip to indicate whether the DC output voltage exceeds a predetermined value. The control circuit is coupled to a control input of the switching element for periodically switching the main current path on and off when the feedback circuit indicates that the output voltage is lower than the predetermined value, and for keeping the main current path in a nonconductive state when the feedback circuit indicates that the output voltage exceeds the predetermined value.
The control circuit will now be described in more detail. A logic AND has a first input receiving periodical set pulses from the oscillator, a second input coupled to the fourth terminal, and an output connected to a set input of the set-reset flip-flop. A further logic AND has a first input connected to a non-inverting output of the set-reset flip-flop, a second input connected to an output of the under-voltage detector, and an output connected to the control input of the switching element. An output of the comparator is coupled to a reset input of the set-reset flip-flop. The set pulses are supplied to the set input of the set-reset flip-flop when the feedback circuit does not pull the fourth terminal low, which is the case when the DC output voltage is lower than a predetermined value. The FET will be switched on when both the set-reset flip-flop is set and the under-voltage detector detects that the voltage across the capacitor at the third terminal is higher than the further reference voltage. The main current path of the FET stays on until the voltage at the first terminal exceeds the first-mentioned reference level causing the set-reset flip-flop to reset. When the DC output voltage exceeds the predetermined level, the optocoupler diode starts emitting light and the optocoupler transistor pulls the fourth terminal to ground, thereby preventing a set of the set-reset flip-flop. Consequently, the main current path of the FET will stay off until the DC output voltage drops below the predetermined value. In this way, an energy-efficient burst mode is obtained, and the switched-mode power supply is only active during short periods of time. However, this switched-mode power supply chip requires four terminals.